The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zone and a process for the making of the same. More particularly, the present invention relates to coated inserts with enhanced properties in applications demanding high edge toughness.
Coated cemented carbide inserts with binder phase enriched surface zone are used today to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone, an extension of the application area for the cutting tool material is obtained.
Methods to make cemented carbide containing WC, cubic phase (gamma-phase) and binder phase with binder phase enriched surface zones are within the technique referred to as gradient sintering and are known through a number of patents and patent applications. According to U.S. Pat. Nos. 4,277,283 and 4,610,931, nitrogen containing additions are used and sintering takes place in vacuum whereas according to U.S. Pat. No. 4,548,786, the nitrogen is added in gas phase. In both cases a binder phase enriched surface zone essentially depleted of cubic phase is obtained. U.S. Pat. No. 4,830,930 describes a binder phase enrichment obtained through decarburization after the sintering whereby a binder phase enrichment is obtained which also contains cubic phase.
In U.S. Pat. No. 4,649,084, nitrogen gas is used in connection with the sintering in order to eliminate a process step and to improve the adhesion of a subsequently deposited oxide coating.
Gradient sintering of cemented carbide inserts according to known technique results, for essentially plane surfaces, in a binder phase enriched surface zone essentially free of cubic phase. In edges and corners, however, a complex superposition of this effect is obtained. The binder phase enriched surface zone in these parts of an insert is generally thinner and the content of cubic phase in a corner area is increased relative to that of an essentially plane surface with a corresponding decrease in binder phase content (FIG. 3). In addition, the cubic phase in said area is more coarse grained than in the interior of the insert (FIG. 1).
However, the edges of a cutting insert have to have a certain radius of the order of 50-100 .mu.m or less in order to be useful. The edge radius is generally made after sintering by an edge rounding operation. In this operation, the thin outermost binder phase enriched zone is completely removed and the hard, brittle area is exposed. As a result, a hard but brittle edge is obtained. Inserts made by gradient sintering according to known technique therefore compared to `straight`, not gradient sintering processes pose an increased risk for brittleness problems in their edges, particularly in applications demanding high edge toughness.
This is particularly the case when sintering according to the teachings of, e.g., U.S. Pat. No. 4,610,931. Also, when using the technique disclosed in Swedish Patent Application no. 9200530-5, which corresponds to U.S. Ser. No. 08/019,701, incorporated herein by reference, essentially the same situation occurs.